The present invention relates to a method of manufacturing and more particularly of heat treating a strip or sheet of Zircaloy 2 or Zircaloy 4. The compositions of these alloys, Zircaloy 2 and Zircaloy 4, are given in ASTM Specification B 352-85, in which these alloys correspond to grades R60804 and R60802, respectively.
Typically, the cases of fuel assemblies for boiling water nuclear reactors are made of Zircaloy 4 or Zircaloy 2.
Two essential properties that are sought are good behavior in terms of nodular corrosion and low deformation under irradiation. To obtain these results, it is essential that the constituent sheets have an adequate metallurgical structure, including fine second-phase precipitates quite close to one another, and an isotropic crystallographic structure.
Structures originating from various methods have been proposed to obtain the distribution of precipitates desired:
a) alpha-type obtained by production methods which after an intermediate quenching from the beta range include only alpha heat treatments, at limited times and temperatures.
b) (alpha+beta) structures, obtained by rapid heating of a sheet by high-frequency induction followed by a rapid chilling, in accordance with French Patent Document B 2,303,865 (equivalent to British Patent 1,537,930, Swedish Patent 7502865, and Swedish Patent 7511523), or obtained by rapid heating of some part, such as a case or a cladding tube, again by electrical induction, this heating being followed by rapid chilling until the alpha range is reached, as described in French Patent 2,302,569 (equivalent to U.S. Pat. No. 4,238,251). In the second method, provision is made for treatments in the beta range up to 825.degree. and 965.degree. C., but no example is given.
In (alpha+beta) heat treatments, it is necessary for the heating to be rapid (within a few seconds), and the cooling also, for example at least 20.degree. C. per second down to 720.degree. C. The conditions taught are not applicable to very long strips, typically more than 100 m long, with sheets for cases then being cut from such a strip.
Applicants have sought to develop a method that is applicable to strips, for uniformly providing good resistance to nodular corrosion and low deformation under irradiation.